If the second or fourth row of keypad buttons also didn't work then it could a continuity issue with pins 3 and 5. Since the keypad does work it should be an issue under the disc. Hopefully the plastic circuit is not worn through. You should be able to visually tell when you open it up.

Like Fdr said, flashbacks are a good source for replacement controllers. Try and pay under $25 for one with two controllers. You can contact atariage user Nurmix about an adapter. When you open your Intellivision you'll see each controller has a 9pin connector but its different than the DE9 connectors on other models or the flashbacks.

I really appreciate the responses! I'll try cleaning the controller first. If that doesn't work, I'll search for an Intellivision Flashback and buy the adapter.

Be VERY careful if you open the controller to attempt to clean the mylar. The printed traces are very fragile, and rubbing too hard with isopropyl alcohol can take sections of the traces off the mylar.

More than likely, the left-right area of the traces are worn on the mylar inset, and replacing it will be the best option. The hand controllers are very easy to disassemble, just 4 screws on the back of each, but when you take the front off be careful to note which way everything goes back together, including how the mylar is folded.

I tried conductive pen to attempt to repair traces on an Atari Lynx mylar controller insert, but had no luck. Anyone actually tried this on an Intellivision mylar insert with positive result?